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Genetic dynamics of E. coli O157:H7 on farms R and X. Both farms had dominant lineage II strains until new Stx2-positive lineage I strains emerged in 1999 on farm R and 1994 on farm X, respectively. The Stx2-positive strain on farm R disappeared and on farm X was modified (stx2::IS genotype) during the following years. This model highlights how Stx2-negative E. coli O157:H7 became prevalent on the farms over time at the strain and genetic levels.
